Electrical lighting systems are very inefficient and can contribute significantly to air conditioning loads, thereby increasing overall electricity costs. For instance, incandescent lights waste about 97% of their electrical energy as heat, and fluorescent lights waste about 90%. In contrast, the sun is a high intensity energy source of freely available sunlight. In order to take advantage of this energy source, sunlight collectors can be made that are small and light enough to be mounted on conventional roofs, walls, window sills and the like, without the need for flashing or any significant structural alterations. Attempts have been made to design such sunlight collectors. U.S. Pat. No. 4,539,625 describes a lighting system for a building interior that utilizes a solar light receiving stack of luminescent concentrators connected to an optical conduit consisting of optical fibers that transmit light to a fixture located in the area to be illuminated. However, the stack and optical conduit are very wide and this poses physical difficulties in installing the system since the stack located on the outside of the building will need to communicate with the optical conduit located in the interior by passing through a similarly wide aperture in the building wall or like barrier between inside and outside.
Additionally, a very wide optical conduit will have limited flexibility and accessability and so may not be able to access locations remote from the solar light receiving stack. On the other hand, a relatively narrow, thin and flexible optical conduit would forseeably pose fewer, if any, problems in installation and accessibility to remote locations.
The solar light receiving stack or collector of U.S. Pat. No. 4,539,625, because of its relatively large width to length ratio, will only have an appreciable contribution from total internal reflection at the top surface, the bottom surface and the end opposite the optical conduit. There is no appreciable contribution from total internal reflection at the collector side edges. It is therefore reliant upon having a correspondingly wide optical conduit.
Furthermore, it is reliably anticipated that due to design flaws, the output colour of light from the system of U.S. Pat. No. 4,539,625 will not be neutral or near neutral.
It is therefore an object of the present invention to provide an improved means for lighting the interior of a building or the like which involves collecting sunlight and transmitting it to the interior of the building or the like.
It is another object of the present invention to provide luminescent solar concentrator systems for lighting interior spaces that cannot be lit by existing daylighting systems, and which provide light into any interior spaces with much less impact on the building fabric (walls, roof, ceiling etc) than existing technologies. In practice, these goals can only be widely achieved if the light from, say, the fluorescent solar concentrator is channelled into the interior via a flexible optical conduit or light guide which does not have a large cross section or large width. Although some inventors of flat panel fluorescent solar concentrator systems have specified the use of a flexible light guide, (for example, see U.S. Pat. No. 4,539,625), various aspects of their design have prevented the economic realisation of this essential component with known materials. It is an important advantage of the present invention that it permits the construction of high performance flexible light guides made from known low cost materials without reducing the amount of light supplied.
The torsional stiffness of a rectangular light guide (ie its resistance to twisting) is proportional to the cube of its width. Thus, wide light guides (as exemplified in U.S. Pat. No. 4,539,625) with only a few elements are extremely difficult to twist and this precludes their use in the lighting of almost all existing buildings. It is, of course, possible to custom design a new building so that wide light guides from fluorescent solar concentrators can convey light to desired parts of the building without any twists. However, doing so places severe constraints on the building design in that one virtually has to design the building around the lighting system. This approach has not found favour with architects or building developers.
The utilisation of a large number of very thin light guides has been proposed in U.S. Pat. No. 4,539,625 as a method of making flexible light guides. However, such a solution requires numerous optical joints between each collector element and the many light guides. While possible in principle, there are many practical difficulties with this approach. For example, efficient operation of a fluorescent solar concentrator system requires that there be an optical joint between the collector and the light guides. Without this joint, performance degrades by an order of 50%. This joint may be made with some type of optical glue material. If the small light guide elements are closely spaced (as is required for a high efficiency), then it is very difficult to eliminate the optical joint material from the side of the light guides. This material causes a high light loss. Surrounding the small light guides with protective sleeves actually tends to make the situation worse, as capillary action will then draw the optical joint material into space between the sleeves and the light guide and it severely reduces the available fraction of light that can be transferred. Because of these and similar problems, it is extremely difficult to avoid unacceptably large light losses when a multiplicity of fibres is used to achieve flexibility. Mass production of such a system would be a challenging exercise. An additional drawback is that very narrow light guides tend to be significantly more expensive per unit cross sectional area than larger ones. (Indeed, below a certain width, the cost per linear meter of light guide is almost independent of the light guide's width.) Thus to transport a given amount of light with numerous small light guides costs considerably more than using a single large light guide.
In principle it might be possible to make a flexible light guide by using a material that has a low enough value of Young's modulus (for a light guide with a given cross section, both torsional stiffness and bending stiffness are inversely proportional to the material's Young's modulus). Unfortunately, the optical and mechanical requirements for the light guide are such that there are no known suitable materials. Many applications of fluorescent solar collector systems require that the loss of light in the light guide be no more than a few percent per meter. This places very stringent limits on the material's optical and mechanical properties. The side surfaces represent a particular challenge. Prior art light guide designs for use with flat luminescent concentrators use a rectangular cross section with very smooth sides and sharp corners. Making light guides of this type in a soft material would be very difficult. Rounding the corners of the light guide might simplify fabrication, but unfortunately this would decrease the amount of light that can be transported. (However, this light loss might be acceptable for some applications.)
A practical, efficient and economic fluorescent solar concentrator system must be designed as an integrated unit. In order to be able to install the system in existing buildings (and new buildings of standard design) the light guides must be flexible. This requires that they must be reasonably narrow. If the light guide is directly connected to the collector (via an optical joint) then the collector must have a similar (narrow) width. The design of the preferred system of the present invention represents a practical way of achieving this important requirement. The aspect ratio of a rectangular fluorescent concentrator may be defined as the ratio of the element's length (measured perpendicular to its output surface) to its width (measured parallel to its output surface). Previous designs have generally employed aspect ratios of less than 0.3. The present invention uses an aspect ratio of more than 4.0 and preferably more than 8.0. This large increase in aspect ratio permits the use of a light guide that is narrow and hence flexible.
Several fluorescent solar concentrator systems based on these principles have been constructed and installed for the purposes of reasonable trial and in secret in an existing building. It was found that systems using 120 mm wide light guides made from 2 mm thick polymethyl methacrylate (a cheap, readily available material with excellent light transmission properties) were wide enough to permit adequate light output, yet flexible enough to permit easy installation.
It is yet another object of the present invention to provide fluorescent solar concentrator systems which achieve a neutral white light output. Indeed, achieving the correct colour balance is often more important than achieving the maximum possible output. With known dyes and a three layer collector stack (of violet, green and red sheets) it is very hard to achieve a good neutral white output while achieving near maximum light output. The present invention overcomes this problem by using, in a preferred form, two sub-sheets A and B arranged end to end (a tandem sheet) at the bottom of the collector stack. The dyes in the two collector sub-sheets A and B are chosen such that the fluorescent emission of the collector sub-sheet furthest from the light guide (B) can pass with low loss through the collector sub-sheet closest to the light guide (A). The two collector sub-sheets A and B are connected by an optical joint.
With most dye pairs, the fluorescent emission of collector sub-sheet A is absorbed by the dye in collector sub-sheet B. With this arrangement, almost all of the trapped fluorescent emission of collector sub-sheet B reaches the light guide, while only about one half of the trapped fluorescent emission of collector sub-sheet A reaches the light guide (the other half enters sub-sheet B where it is absorbed and so boosts that sub-sheet's output).
For a given collector length, there are five variables to adjust (four dye concentrations and the ratio of the lengths of the collector sub-sheets) in the aforementioned collector stack rather than the three variables in a three layer collector stack not having any sub-sheets (the three dye concentrations). This makes it much easier to achieve a good neutral white output while achieving an acceptable luminous output. In many situations, this improvement in colour more than compensates for any reduction in luminous output. Indeed, with some dye pairs, using a tandem sheet actually increases the system's luminous output.